Safe cast for uint

[monlu92]

Negative integers would convert into big uint's instead of failing.
I have added tests covering the case.

[coveralls]

coverage: 79.191% (+0.06%) from 79.131% when pulling 78f39d6 on monlu92:master into dbb6117 on 99designs:master.

[StevenACoffman]

Looks like some lint and test failures. Can you try this:

go generate ./...; cd _examples; go generate ./...; cd ..; go test ./...; cd _examples; go test ./...

[chadxzs]

Nice small improvement @monlu92! Did you run into this as a bug in the wild? Or find it through some other means?

[monlu92]

@chadxzs thanks! I did just find it in the wild when making some graphQL resolvers, great library otherwise though :)

[StevenACoffman]

I like this functionality, but it is implemented using golang.org/x/exp per https://pkg.go.dev/golang.org/x/exp#section-readme:

Warning: Packages here are experimental and unreliable. Some may one day be promoted to the main repository or other subrepository, or they may be modified arbitrarily or even disappear altogether.

Is it possible to implement this without dependending on an unreliable package? Also, there are merge conflicts, so it needs to be rebased.